Chapter 12 Questions

Which of the following best approximates the value of ΔG° for this reaction: 2 Al + 3 Cu2+ → 2 Al2+ + 3 Cu? A) -(12)(96,500)J B) -(6)(96,500)J C) +(6)(96,500)J D) +(12)(96,500)J

A) -(12)(96,500)J The half reactions are 2(Al → Al3+ + 3e-) E° = +1.67V 3(Cu2+ + 2e- → Cu) E° = 0.34V The overall cell voltage is E° = 2.01V = 2V. Because the number of electrons transferred is n = 2 x 3 = 6, the equation is: ΔG° = - (6)(96,500)(2)J = -(12)(96,500)J

A piece of steel is the cathode in a hot solution of chromic acid (H2CrO4) to electroplate it with chromium metal. How much chromium would be deposited onto the steel after 48,250 C of electricity was forced through the cell? A) 1/12 mol B) 1/6 mol C) 1/4 mol D) 3 mol

A) 1/12 mol 48,250J is equal to 1/2 faraday (96,500 C/mol). This is equivalent to a half mole of electrons. In the molecule H2CrO4, chromium is in a +6 oxidation state. So, from the stoichiometry of the reaction Cr6+ + 6e- → Cr, we see that for every 6 moles of electrons gained, we get 1 mole of Cr metal. Another way of looking at this is to say that for every 1 mole of electrons gained, we get just 1/6 mole of Cr metal. Therefore, if we have a supply of 1/2 mol of electrons, we'll produce (1/6)(1/12) = 1/12 mol of Cr.

A galvanic cell is set to operate at standard conditions. If one electrode is made of magnesium and the other is made of copper, then the magnesium electrode will serve as the: (2 checks) A) anode and be the site of oxidation B) anode and be the site of reduction C) cathode and be the site of oxidation D) cathode and be the site of reduction Mg2+ + 2e- → Mg E° (volts) = -2.36 Cu2+ + 2e- → Cu E° (volts) = 0.34

A) anode and be the site of oxidation

Typical dry cell battery contains a zinc anode and a carbon cathode and produce a potential difference of 1.5 V. Given that many electronic devices require additional voltage, which of the following would result in an overall increase in voltage? I. Doubling the quantity of Zn (s) II. Placing two batteries in parallel III. Replacing Zn(s) with Na(s) A) I only B) III only C) I and II only D) II and III only

B) III only Increasing reagent quantity has no effect on voltage and placing batteries in parallel would leave the voltage unchanged. Sodium is a better reducing agent due to its lower ionization energy and tendency to give up an electron.

High valent metals (those with large, positive oxidation states) are often used as strong oxidizing agents. Which of the following compounds would have the most positive reduction potential vs a standard hydrogen electrode? A) FeCl3 B) OsO4 C) Zn(NO3)Cl D) W(CO)6

B) OsO4 A large, positive reduction potential indicates a strong tendency to be reduced, and hence ability to act as an oxidizing agent. The question states that high-valent metals act as strong oxidizing agents. Examining the oxidation states of the metals in question, we see that Fe = +3, Os = +8, Zn = +2, and W = 0. Therefore. since Os bears the largest positive oxidation state, we know that it is the strongest oxidizing agent.

To give "white gold" a white appearance, it is plated with rhodium by immersion in a rhodium sulfate solution (Rh2(SO4)3(aq)). Provided with a current of 2.0 A, how long must a 3.0 g white gold broach be immersed to plate 3.0 x 10^-5 g of rhodium? A) 0.0009 s B) 0.0098 s C) 0.042 s D) 0.56 s

C) 0.042 s Since current (I) = Q/t, you can find the time by dividing charge by current. 1 mol e- = 96,500 C

A galvanic cell is constructed from two half-cells of platinum and iron. The half-reactions for these two elements are provided as follows: (1 check) Pt2+(aq) + 2e- → Pt (s) E° = +1.20 V Fe3+(aq) + 3e- → Fe(s) E° = -0.036 V Which of the following statements is true about the galvanic cell? A) E° = 1.164 V, and Pt2+ is the reducing agent B) E° = 1.164 V, and Fe3+ is the reducing agent C) E° = 1.236 V, and Pt2+ is the oxidizing agent D) E° = 1.236 V, and Fe3+ is the oxidizing agent

C) E° = 1.236 V, and Pt2+ is the oxidizing agent Pt2+ is reduced (it gains electrons from the Fe that is oxidized), so it is therefore the oxidizing agent.

Which of the following best describes the difference between a galvanic cell and an electrolytic cell? (2 checks) A) In a galvanic cell, the anode is the site of oxidation, whereas in an electrolytic cell the anode is the site of reduction B) In a galvanic cell, the cathode is the negative electrode, whereas in an electrolytic cell the cathode is the positive electrode. C) In a galvanic cell, spontaneous reactions generate a current, whereas in an electrolytic cell a current forces nonspontaneous reactions to occur. D) In a galvanic cell, the electrons flow from anode to cathode. whereas in an electrolytic cell the electrons flow from cathode to anode.

C) In a galvanic cell, spontaneous reactions generate a current, whereas in an electrolytic cell a current forces nonspontaneous reactions to occur.

Of the following, which is the strongest reducing agent? A) Zn Zn2+ + 2e- → Zn E° (volts) = -0.76V B) Fe Fe2+ + 2e- → Fe E° (volts) = -0.44V C) Pd Pd2+ + 2e- → Pd E° (volts) = 0.99 D) Pb2+

C) Pd In order for Pd2+ to be a reducing agent, it would have to be oxidized - that is, lose more electrons, which is not likely for a cation to get further oxidized.

An electrochemical cell is constructed using two inert electrodes in one chamber with an inert electrolyte. The binary compound ICl is dissolved in the electrolyte, current is applied, and I2 and Cl2 are produced. Which of the following statements is true? A) Cl2 was produced by reduction at the cathode B) I2 was produced by oxidation at the cathode C) Cl2 was produced by oxidation at the cathode D) I2 was produced by reduction at the cathode

D) I2 was produced by reduction at the cathode In the compound ICl, I has an +1 oxidation state. and Cl has a -1 oxidation state owing to the greater electronegativity of Cl. Therefore, production of Cl2 must be an oxidation, and production of I2 must be a reduction.

The final products of the electrolysis of aqueous NaCl are most likely: (1 check) A) Na(s) and C2(g) B) HOCl(aq) and Na(s) C) Na(s) and O2(g) D) NaOH(aq) and HOCl(aq)

D) NaOH(aq) and HOCl(aq) Choices A, B, and C list metallic sodium as a final product. That's a problem because we're in an aqueous medium, and we know that sodium metal reacts violently in water to form NaOH and hydrogen gas and fire.

Given the following reactions: Pb2+ + 2e- → Pb(s) E° = -0.13V Fe(s) → Fe2+ + 2e- E° = 0.45 V Which of the following is true? A) Pb(s) is a better reductant than Fe(s) B) Fe(s) is a worse reductant than Pb(s) C) Fe2+ is a better oxidant than Pb2+ D) Pb2+ is a better oxidant than Fe2+

D) Pb2+ is a better oxidant than Fe2+ Pb2+ + 2e- → Pb(s) E° = -0.13V Fe2+ + 2e- → Fe(s) E° = -0.45 V

Which of he following best characterizes the spontaneous half-reaction below under standard conditions? (2 checks) Pd2+ + 2e- → Pd A) ΔG° > 0 and E° < 0 B) ΔG° < 0 and E° < 0 C) ΔG° > 0 and E° > 0 D) ΔG° < 0 and E° > 0

D) ΔG° < 0 and E° > 0